Marathon-related ECG Exasperation

The following two ECGs were taken from a 36-year old marathon runner presenting to the ED with acute gastroenteritis. He was completely asymptomatic at the time the ECGs were taken, with normal blood pressure and no chest pain, palpitations or dizziness. Electrolytes were normal.

Isorhythmic AV dissociation: AV dissociation with sinus and ventricular complexes occurring at similar rates, unlike 3rd degree heart block where the atrial rate is usually faster than the ventricular rate. Isorhythmic AV dissociation is usually due to functional block at the AV node from retrogradely conducted ventricular impulses (“interference-dissociation”), which leaves the AV node refractory to the anterograde sinus impulses.

Sinus arrhythmia: Sinus rhythm in which the PP interval varies by 0.16 s or more.

The likely mechanism of the AIVR in this patient is enhanced automaticity of an ectopic pacemaker in the ventricles coupled with sinus bradycardia and sinus arrhythmia due to athletic training.

There are pacemaker cells at various sites throughout the conducting system, with each site capable of independently sustaining the heart rhythm. The rate of spontaneousdepolarisation of pacemaker cells decreases down the conducting system:

SA node (60-100 bpm)

Atria (< 60 bpm)

AV node (40-60 bpm)

Ventricles (20-40 bpm)

Under normal conditions, subsidiary pacemakers are suppressed by the more rapid impulses from the sinus node. AIVR occurs when the rate of an ectopic ventricular pacemaker exceeds that of the sinus node.

AIVR is a benign rhythm in most settings and does not usually require treatment.

Administration of anti-arrhythmics may cause precipitous haemodynamic deterioration and should be avoided.

Treat the underlying cause:

correct electrolytes

restore myocardial perfusion

give Digibind in the case of digoxin toxicity.

Patients with low-cardiac-output states (e.g. dilated cardiomyopathy) may benefit from restoration of AV synchrony to restore atrial ’kick’ – in this case atropine may be used to increase sinus rate and AV conduction

About Edward Burns

Ed Burns is an Emergency Physician working in Prehospital & Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education. Ed is the force behind the LITFL ECG library | + Edward Burns | @edjamesburns